Liquid crystal display devices are used in various household electrical appliances, measuring instruments, automobile panels, word processors, electronic notebooks, printers, computers, and television sets, as well as clocks and watches and electronic calculators. Typical liquid crystal display modes include twisted nematic (TN), super-twisted nematic (STN), dynamic light scattering (DS), guest-host (GH), in-plane switching (IPS), optically compensated birefringence (OCB), electrically controlled birefringence (ECB), vertical alignment (VA), and color super homeotropic (CSH). Regarding the drive system, multiplex driving becomes common instead of conventional static driving, and passive-matrix and recent active-matrix (AM) driven by thin-film transistors (TFT) and thin-film diodes (TFD) become the mainstream.
As illustrated in FIG. 1, a typical color liquid crystal display device includes two substrates (1) each having an alignment film (4). A transparent electrode layer (3a) and a color filter layer (2) are disposed between the alignment film and one of the substrates. The transparent electrode layer (3a) acts as a common electrode. A pixel electrode layer (3b) is disposed between the alignment film and the other of the substrates. These substrates are disposed such that the alignment films face each other with a liquid crystal layer (5) interposed therebetween.
The color filter layer is a color filter composed of a black matrix, a red color layer (R), a green color layer (G), a blue color layer (B), and an optional yellow color layer (Y).
Impurities in the liquid crystal material of the liquid crystal layer are strictly controlled because these impurities have a great influence on the electrical characteristics of display devices. The influence of impurities in the materials of the alignment films on the characteristics of liquid crystal display devices is also being studied. The alignment films are in direct contact with the liquid crystal layer. It is already known that impurities in the alignment films move into the liquid crystal layer and have an influence on the electrical characteristics of the liquid crystal layer.
In the same manner as in the alignment film materials, impurities in the materials of the color filter layer, such as organic pigments, may have an influence on the liquid crystal layer. However, it has been believed that the alignment film and the transparent electrode between the color filter layer and the liquid crystal layer greatly reduce the direct influence of the color filter layer on the liquid crystal layer as compared with the alignment film materials. However, the alignment film generally has a thickness as small as 0.1 μm or less, and the transparent electrode used as a common electrode adjacent to the color filter layer generally has a thickness of 0.5 μm or less even when the thickness is increased to achieve high electric conductivity. Thus, the color filter layer is not necessarily completely separated from the liquid crystal layer. Impurities in the color filter layer may cause display defects, such as white spots, variations in alignment, and burn-in, due to a decrease in the voltage holding ratio (VHR) of the liquid crystal layer through the alignment film and the transparent electrode in liquid crystal display devices. A method for controlling the elution of impurities into liquid crystals by using a pigment having a rate of extraction by ethyl formate not more than a particular value (Patent Literature 1) and a method for controlling the elution of impurities into liquid crystals by specifying a pigment in a blue color layer (Patent Literature 2) have been studied as methods for preventing display defects caused by impurities contained in pigments of color filters. However, there is no significant difference between these methods and a simple reduction of impurities in pigments. Even with recent advances in pigment purification technology, these methods are insufficient to prevent display defects.
Focusing on the relationship between an organic impurity in a color filter and a liquid crystal composition, in a method disclosed in Patent Literature 3, the insolubility of the organic impurity in a liquid crystal layer is represented by the hydrophobicity parameter of liquid crystal molecules in the liquid crystal layer, and the hydrophobicity parameter has at least a certain value; or on the basis of the correlation between the hydrophobicity parameter and a —OCF3 group at an end of liquid crystal molecules, a liquid crystal compound having a —OCF3 group at an end of its liquid crystal molecules constitutes at least a certain percentage of the liquid crystal composition.
However, also in this disclosure in the cited literature, it is essential to reduce the influence of impurities in a pigment on the liquid crystal layer. In particular, no study has been performed on the direct relationship between the structure of a coloring material, such as a dye or pigment, in a color filter and the structure of a liquid crystal material. Thus, the problem of display defects has not been solved in advanced liquid crystal display devices.